1. Field of the Invention
This invention relates to a semiconductor transducer comprising a semiconductor strain gauge bonded to a strain-yielding body which yields a strain when a force or displacement, is imparted thereto.
2. Description of the Prior Art
Semiconductor transducers are widely used as a means for converting a force, pressure, displacement or the like into an electrical signal. FIG. 1 shows an example of such a semiconductor transducer. The semiconductor transducer shown in FIG. 1 comprises a cantilever 10, a pair of semiconductor strain gauge units 21 and 22, and a die member 30 fixedly supporting the cantilever 10. The semiconductor strain gauge units 21 and 22 are bonded by associated layers 41 and 42 of a bonding material to the upper and lower surfaces respectively of the cantilever 10 adjacent to the base portion 50 of the cantilever 10. The cantilever 10 is fixed at its base portion 50 to the die member 30 by fixing means such as screws 60. When a force W or a displacement input is imparted to the tip or free end of the cantilever 10, the cantilever 10 is deflected, and a tensile strain or a compressive strain proportional to the input is produced in the semiconductor strain gauge unit 21 or 22. Therefore, an electrical signal proportional to the force w or displacement input imparted to the free end of the cantilever 10 can be obtained when the semiconductor strain gauge units 21 and 22 bonded to the upper and lower surfaces of the cantilever 10 are arranged to constitute two arms of a Wheatstone bridge circuit.
In the semiconductor transducer of this kind, the semiconductor strain gauge units 21 and 22 bonded to the cantilever 10 have a low coefficient of thermal expansion of about 3.0.times.10.sup.-6 deg.sup.-1 C, and a coefficient of thermal expansion approximately equal to this value is also required for the material of the cantilever 10. Thus, in the prior art semiconductor transducer, Fernico (an alloy of iron, nickel and cobalt) has been generally employed as the material of the cantilever 10.
Such a prior art semiconductor transducer is disclosed in, for example, Japanese Patent Application Kokai (Laid-Open) No. 51-22477 entitled "SEMICONDUCTOR STRAIN GAUGE". In the cited application, cold working is applied to the Fernico material so that, when the semiconductor strain gauge units exhibiting the piezo-resistive effect are bonded to the cantilever of Fernico material, the thermal strain due to the slight difference between the coefficients of thermal expansion of these materials may not be added as an error to the strain value detected by the semiconductor transducer. However, due to the fact that the Fernico material has a poor elastic property compared with that of elastic materials commonly employed and has also a low mechanical strength, the operating characteristics of the semiconductor transducer has been limited by the mechanical property of the Fernico material. Especially, due to the considerable hysteresis and resultant bad linearity of the Fernico material, the operating characteristic of the semiconductor transducer has tended to be degraded with the increase in the range of strain produced in the cantilever, and it has been unable to expect a transducer accuracy higher than 0.5%. Further, the accurately detectable strain range has been limited to about 500.times.10.sup.-6 in order that the semiconductor transducer can operate with an accuracy of the order of 0.5% when the factors including the linearity, hysteresis and zero-point stability are collectively taken into account.